So to speculate further ... did these 4 immediately locate alternative food, water and roosting locations! so quickly? If they weren't taken by predators, where are they? I recall you suspect they may have been attracted to the Zoo flock? I find it hard to believe they have gone very far - but I've been wrong almost every time.
I can't really see why you should lose 4 of 7.
I can't really see why you should lose 4 of 7.